The present invention relates generally to digital audio broadcasting (DAB) and other techniques for transmitting information in a communication system.
Proposed systems for digital audio broadcasting (DAB) in the AM and FM radio bands are expected to provide improved-quality audio, data services, and more robust coverage than existing analog transmissions. These all-digital systems are often referred to as in-band on-channel (IBOC) DAB systems. However, until such time as a transition to all-digital DAB can be achieved, broadcasters require an intermediate solution in which the analog and digital signals can be transmitted simultaneously within the same licensed band. Such systems are typically referred to as hybrid in-band on-channel (HIBOC) DAB systems, and are currently being developed for both the AM and FM radio bands.
The above-noted DAB systems may be designed such that partial-band interference exists in certain areas of the system or under certain operating conditions. For example, it may be desirable in these systems to allow digital sidebands of adjacent carriers to overlap to a limited extent in specified areas or operating conditions, such that higher bit rates can be supported.
Techniques for alleviating the effects of partial-band interference and otherwise improving the performance of IBOC and HIBOC DAB systems are described in U.S. patent application Ser. No. 09/290,819 filed Apr. 13, 1999 in the name of inventors D. Mansour, D. Sinha and C.-E. W. Sundberg and entitled xe2x80x9cMultistream In-band On-channel Systems,xe2x80x9d U.S. patent application Ser. No. 09/322,848 filed May 28, 1999 in the name of inventors J. N. Laneman and C.- E. W. Sundberg and entitled xe2x80x9cSoft Selection Combining Based on Successive Erasures of Frequency Band Components in a Communication System,xe2x80x9d U.S. patent application Ser. No. 09/464,042 filed Dec. 15, 1999 in the name of inventors J. N. Laneman, D. Sinha, C.-E. W. Sundberg and J. W. Tracey and entitled xe2x80x9cError Screening Based on Code and Control Information Consistency in a Communication System,xe2x80x9d and U.S. patent application Ser. No. 09/464,043 filed Dec. 15, 1999 in the name of inventors J.N. Laneman, D. Sinha and C.-E. W. Sundberg and entitled xe2x80x9cChannel Code Configurations for Digital Audio Broadcasting Systems and Other Types of Communication Systems,xe2x80x9d all of which are assigned to the assignee of the present invention and incorporated by reference herein.
A DAB system may use as a channel code a complementary punctured pair convolutional (CPPC) code with optimum bit placement (OBP) in the digital sidebands. A number of CPPC codes and OBP techniques suitable for use in IBOC and HIBOC systems are described in, e.g., U.S. patent application Ser. No. 09/217,655, filed Dec. 21, 1998 in the name of inventors B. Chen and C.-E. W. Sundberg and entitled xe2x80x9cOptimal Complementary Punctured Convolutional Codes,xe2x80x9d which is assigned to the assignee of the present invention and incorporated by reference herein. The bit placement may be optimized, e.g., for a case in which the outermost frequency components are expected to be the sideband components most susceptible to first adjacent interference.
Despite the considerable advances provided by the techniques described in the above-cited U.S. Patent Applications, a need nonetheless remains for further performance improvements in DAB systems and other systems subject to partial-band interference.
The present invention provides methods and apparatus for channel coding in digital audio broadcasting (DAB) systems or other types of digital communication systems, so as to provide enhanced performance relative to conventional systems in the presence of partial-band interference. In accordance with one aspect of the invention, digital information is processed for transmission in a communication system using a hidden puncturing technique. The digital information is encoded using a channel code having at least one puncturing pattern associated therewith. The puncturing pattern is preferably a rate compatible puncturing pattern. There is no puncturing of the channel code applied at a transmitter side of the system. Instead, the resulting channel coded bits are assigned to different sections of a digital sideband or other portion of a frequency spectrum of the system, based at least in part on the puncturing pattern. The coded channel bits are then transmitted through the system in the assigned sections, and can be decoded at different code rates in a receiver of the system depending on which of the sections are used in the decoding process. By allowing different sections to be erased and the decoding to proceed at a corresponding higher code rate, the invention alleviates the effects of partial-band interference in the system.
In an illustrative embodiment of the invention, the digital information is transmitted using both an inner channel code and an outer channel code. The above-described assignment of channel coded bits to the different sections of the portion of the frequency spectrum is then applied to the inner channel coded bits. An error indicator generated using the outer code may then be utilized in the receiver to determine which of the sections of the portion of the frequency spectrum should be decoded as part of the decoding process.
In accordance with another aspect of the invention, the above-noted decoding process may be implemented by first decoding the channel code using all of the sections. Then, if a cyclic redundancy check (CRC) or other specified error criteria is not satisfied, one or more of the sections are erased and the decoding is repeated using the remaining subset of the sections at a corresponding higher code rate. This process may be repeated until the specified error criteria is satisfied or all of the possible code rates have been decoded. If the specified error criteria remains unsatisfied after all of the possible code rates have been decoded, an error mitigation algorithm in a source decoder may be triggered.
An advantage of the hidden puncturing technique of the present invention is that it does not require estimates of partial-band interference levels. Instead, the above-noted CRC or another type of error indicator may be used as an indicator of the presence of such interference.
The invention is particularly well-suited for use with convolutional codes or Turbo codes, but could also be used with other types of channel codes.
The invention can be applied to any type of digital information, including, for example, audio, data, video and image information, as well as various combinations thereof. In addition, the invention may be implemented in numerous applications other than FM and AM HIBOC DAB systems, such as Internet and satellite broadcasting systems, systems for simultaneous delivery of audio and data, etc.